Process baths in the surface treatment are subject to modifications due to the intended mass conversion, the dragging-in of water and impurities, the dragging-out of electrolyte and due to other influences such as evaporation, anodic or cathodic secondary reaction, the capture of components from the air and false dosages. This is the reason why the used process solutions have to be regenerated continuously or in certain intervals, if a new preparation and the dump of the old process solution related therewith shall be avoided. Very different methods for removing individual contraries from process solutions, in particular from electrolytes, are known from the state of the art. Herein, in particular filtration, oxidation, extraction as well as dialysis methods have to be mentioned.
The mentioned methods can be respectively subdivided in different sub-methods. Thus, it is known to filter the process solutions for removing particles such as metal flakes, dust particles, colloids, microorganisms, anode sludge etc. Herein, the process solutions are filtered by means of suitable filters, if necessary, using filter aids such as celite or active carbon, in order to remove the contraries from the process solution. It is a drawback of this method that a filter cake is formed on the filter in the course of the filtration, which filter cake can lead to an increased flow resistance in front of the filter and finally to a clogging-up of the filter. It is possible to maintain the flow resistance of the filter device on a constant value by means of extensive industrial instrumentation, for example by using band filters.
The contraries that can be removed by means of filtration depend on the design of the filter with respect to the size of the pores and the selection of corresponding filter aids.
One embodiment of the filtration relates to the micro- or ultra-filtration using suitable membrane filters. The membrane filters used herein have very small pore sizes with a high retaining power, but disadvantageously lead to a high flow resistance. The micro- or ultra-filtration is mainly used for degreasing baths or acid or alkaline pickling baths.
It is a common aspect of the filtration methods that parasitic substances that are completely dissolved in the process solutions cannot be removed from the process solutions by this technique.
However, many process solutions used in the surface treatment comprise organic components that are decomposed in the course of the treatment process and finally form decomposition products which interfere with the process chemistry. These interfering decomposition products are often completely soluble in the process solution and cannot be captured by means of filtration methods.
A method for removing these organic decomposition products is the oxidation of the decomposition products by means of for example UV-H2O2 oxidation.
Herein, at least partial volumes of a process bath, for example of a bright nickel electrolyte, are treated with UV/H2O2. The organic components of the electrolyte are discontinuously oxidized outside the bath. All organic compounds, decomposition products and also the acting brighteners and wetting agents are oxidized. In the case of a bright nickel electrolyte the end product of such a treatment is a Watts-type base preparation which can be again admixed to the process bath. The acting additives such as brighteners and wetting agents, which have also been oxidized during the UV/H2O2 oxidation, have to be correspondingly dosed again. Besides this drawback, the UV/H2O2 oxidation is not able to remove inorganic contraries, such as for example foreign ions which interfere with ions being deposited, from the process solutions.
Another possibility of the process solution purification is the liquid-liquid extraction with suitable liquid extraction agents. Herein, impurities are removed from the carrier liquid, for example a galvanic electrolyte to be purified, by means of a liquid extraction agent. The transport of substance from one fluid into the other that takes place herein results from the different solubilities of the substance in the fluids and the prevailing concentration gradient.
Herein, the extraction agent and the carrier liquid should be as insoluble with respect to each other as possible in order to assure a good separation and simultaneously carry over as less solvent as possible. In practice, one of the two phases is aqueous and the other one is an organic solvent or a solution of extraction agents in an organic solvent.
Since the extraction is a distribution between two non-mixable phases, the exchange and the establishment of equilibrium are realized via the interphase. A great interphase accelerates the establishment of equilibrium. Therefore, extractions on big technical scale are realized according to the mixer-settler principle. Herein, extraction agent and charged carrier liquid (electrolyte) are mixed with each other (mixer) in a first chamber. The mixture gets into the so called settler, the sedimentation chamber, via a gate of a dam. Here, the phases can separate again. The separated phases are then separately removed from the chamber. If the extraction result does not yet meet the requirements, several mixer-settler units can be placed subsequently.
The condition for a successful use of the liquid-liquid extraction is that a solvent is found which selectively dissolves the impurity to be removed and which can be easily separated again, and that the small residual quantities remaining in the electrolyte do not perturb the application aim of the process solution.
One embodiment of the liquid-liquid extraction is the membrane-supported extraction by means of hollow fibre modules. Herein, the organic extraction agent and the carrier liquid (electrolyte) are separated from each other by a porous membrane. In case of a microporous hydrophobic membrane, the organic phase will spontaneously wet the membrane and try to get through the pores to the other side of the membrane. This breaking-through can be avoided by a low overpressure on the side of the carrier liquid. The intersection between aqueous and organic phase can thus be immobilised in the pore mouth. The driving force of the substance exchange is here also the concentration gradient. The liquids are passed along both sides of the membrane. Herein, the flow rate of both phases can be varied in a wide range. Advantageously, also systems having the tendency to form emulsions can be treated with the hollow fibre supported membrane liquid-liquid extraction. Furthermore, a corresponding device for the hollow fibre module supported liquid-liquid extraction clearly requires less mobile parts in comparison to the extraction according to the mixer-settler principle.
A common aspect of the liquid-liquid extraction methods is that they are limited to corresponding non-mixable extraction/carrier liquid systems and usually no inorganic contraries, such as for example foreign ions, can be removed from the process solutions by means of the liquid-liquid extraction.
Other methods for the purification of process solutions, which are known from the state of the art, are the dialysis methods such as diffusion dialysis or electrodialysis. As in the extraction, in the ion-selective dialysis a concentration gradient is used for the substance transport between the phases, but here hydraulically impervious ion exchange membranes are used.
The centre of such a device is composed of membranes which are held together like a filter press. The anion-selective membranes theoretically retain all cations except the hydrogen ions, which migrate through the membrane due to their small size and their high mobility. The anions and the hydronium ions diffuse simultaneously. Therefore, only strongly dissociating acids offer good conditions for a use.
The process solution to be purified and the dialysis solution to be used, for example water, usually flow through the membrane stack according to the countercurrent principle. The two liquids are separated from each other by membranes and frames that enable the distribution in an alternating manner.
Besides anion-selective membranes, also cation-selective membranes or combinations of anion- and cation-selective membranes can be used.
One embodiment of the dialysis is the electrodialysis which is an electrochemical process, in which ionic components are removed from a solution or exchanged and, if necessary, concentrated by means of ion exchange membranes and the driving force of an electric field.
Herein, the created electric field can reinforce or also reverse the substance transport in the direction of the concentration gradient.
Electrodialysis units are able to also selectively separate ions that are dissolved in the process solutions. Disadvantageously, the used membranes are sensitive and, in particular in case of the electrodialysis, the method is related with a high energy demand.
Besides the already described methods, it is known from DE 43 28 876 A1 as well as from DE 43 18 793 A1 to use an adsorbent polymer for the elimination of organic contraries. Herein, the organic contraries are at least partially adsorbed on the surface of adsorbent polymers and thus removed from the process solution. Advantageously, such a method can be carried out continuously and the used adsorbent polymer can be regenerated by means of suitable solutions, for example a hydrogen peroxide bearing oxidation solution.
However, a drawback of this method is that the adsorbent polymer has to be adapted to the respective organic contraries to be removed and only removes this one very selectively. Furthermore, inorganic contraries such as for example foreign ions cannot be removed by means of this technique.